Most tech startup boards consist of the following expertise areas: Product & Business Strategy, Growth, Tech and Operations.
In the first eight months, a startup's goal would be buildling the right product to satisfy users' needs, wants and demands. This involves iterative product experiements where user feedbacks are fed back into the product development loop. At this stage, product = business model for the startup. Some startups have a more technical prodcut where bespoke softwares have to be built. For these startups, having a tech advisor for reality check purposes would be useful.
The advisors would fill the skills gap of the founding team. For example, a technical founding team may want advisors in growth/operations area to help build traction channels.
Most early-stage startup advisory boards have three to five members. Any less, you don't get diversity in opinions and skills; any more, you may face the 'too many cooks in the kitchecn' problem.